There is known a printer that performs multi-color printing on an outer circumference surface of beverage cans including beer cans in which a drive gear is designed to drive a plurality of plate cylinders corresponding to each of the colors.
In the above-mentioned printer, plate cylinder drive units that transmit rotation of the drive gear to the plate cylinders are provided for respective plate cylinders. Each of the plate cylinder drive units includes a plate cylinder drive shaft rotatably supported on a frame of the printer and rotating with the plate cylinder, and a driven gear member fixed to the plate cylinder drive shaft. The driven gear member includes a driven gear that meshes with the drive gear. In multi-color printers, relative position in the rotational direction (rotational phase) between the plate cylinder drive shaft and the driven gear member is required to be adjusted for register. For fine register, the printer is designed such that the drive gear and the driven gears are helical gears and that the driven gear member is axially moved with respect to the plate cylinder drive shaft to allow axial movement of the driven gear with respect to the drive gear. Further, for rough register, the printer is designed such that the driven gear member is rotatable with respect to the plate cylinder drive shaft and that, during printing, the driven gear member is fixed to the plate cylinder drive shaft and to be nonrotatable relative to the plate cylinder drive shaft by a suitable whirl-stop means.
In printers, eliminating backlash between the drive gear and the driven gear is important to increase rotational accuracy of the plate cylinder and perform high quality printing.
As a structure to eliminate the backlash of the helical gear, there is known a printer in which a backlash eliminating helical gear, as a second driven gear, is used, with the second driven gear being stacked with the first driven gear. One typical example is a structure that a plurality of guide bars each of which is fixed to one side of the first driven gear support the second driven gear in an axially movable manner, and a spring biases the second driven gear toward the first driven gear to bring the teeth of both driven gears in pressure contact with those of the drive gear. At this time, to reliably bring the teeth of the both driven gears in pressure contact with those of the drive gear, clearance is formed between the both driven gears in an axial direction. Because the teeth of the helical gear are provided at an angle, not only reaction force in the circumferential direction but also that in the axial direction is generated at the meshing portion. The drive gear meshes with the driven gear only at one location in the circumferential direction. Therefore, when the second gear starts tilting and keeps rotating in the tilted state, rotation of the driven gear member fails to rotate smoothly and rotational accuracy of the plate cylinder is reduced, resulting in deteriorated printing quality.